Engine exhaust temperature regulation

ABSTRACT

In one example, a method of operating an engine in a vehicle is described. The method comprises delivering a first substance to a cylinder of the engine from a first injector; delivering a second substance to the cylinder of the engine from a second injector, where the second substance has a greater heat of vaporization than the first substance; and increasing injection of the second substance responsive to an exhaust over-temperature condition.

FIELD

The present description relates to a method for estimating andcontrolling exhaust gas temperature settings for an internal combustionengine operating with a variety of fuels of varying composition and fueldelivery.

BACKGROUND AND SUMMARY

Engines may use various forms of fuel delivery to provide a desiredamount of fuel for combustion in each cylinder. One type of fuelinjection, or delivery, uses a port injector for each cylinder todeliver fuel to respective cylinders. Another type of fuel injectionuses a direct injector for each cylinder. Engines have also beendescribed using more than one injector to provide fuel to a singlecylinder in an attempt to improve engine performance.

One such example is described in US 2007/0215110 wherein a flexiblemultiple-fuel engine is described using both port and direct injection,where different fuel types are provided to the injectors. For example,direct injection of ethanol may be used with port injected gasoline. Thedirect injection of ethanol provides improved charge cooling (due toethanol's higher heat of vaporization) and thus improved knocksuppression. One embodiment controls fuel injection responsive to avariety of temperature-related engine operating conditions during enginestart-up. For example, during start-up, fuel injection may be variedamong the different injection locations to provide improved emissioncontrol and starting of combustion.

However, the inventors have herein recognized a potential issue withsuch an approach. For example, while different fuel adjustments canbetter accommodate warming and start-up conditions, there may also beover-temperature conditions at non-starting conditions. In other words,catalyst over-temperature is generally not an issue during starting.Rather, during towing and/or other high load conditions over varyingterrain, catalyst over-temperature conditions may occur and degrade thecatalyst materials.

In one example, the above issues may be addressed by a method ofoperating an engine in a vehicle, the method comprising: delivering afirst substance to a cylinder of the engine from a first injector;delivering a second substance to the cylinder of the engine from asecond injector, where the second substance has a greater heat ofvaporization than the first substance; and increasing injection of thesecond substance responsive to an exhaust over-temperature condition.

In this way, it is possible to address exhaust over-temperatureconditions by preferentially utilizing the increased heat ofvaporization of the second fuel. Such an operation may be especiallyadvantageous when the second substance is directly injected into thecylinder, since the fuel spray may not contact metal surfaces of theengine, so virtually all the heat of vaporization is provided by theair-fuel mixture, thus reducing exhaust gas temperature in addition tocombustion temperature. Additionally, such an operation may beparticularly useful when exhaust equivalence ratio is maintained nearthe stoichiometric ratio, since emission impacts of catalyst temperatureprotection may be reduced.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example embodiment of a combustion chamber operatingwith a plurality of fuel injector options.

FIG. 2 shows a high level flow chart for engine running operationsaccording to the present disclosure.

DETAILED DESCRIPTION

FIG. 1 depicts an example embodiment of a combustion chamber or cylinderof internal combustion engine 10. Engine 10 may be controlled at leastpartially by a control system including controller 12 and by input froma vehicle operator 130 via an input device 132. In this example, inputdevice 132 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP. Cylinder (hereinalso “combustion chamber’) 14 of engine 10 may include combustionchamber walls 136 with piston 138 positioned therein. Piston 138 may becoupled to crankshaft 140 so that reciprocating motion of the piston istranslated into rotational motion of the crankshaft. Crankshaft 140 maybe coupled to at least one drive wheel of the passenger vehicle via atransmission system. Further, a starter motor may be coupled tocrankshaft 140 via a flywheel to enable a starting operation of engine10.

Cylinder 14 can receive intake air via a series of intake air passages142, 144, and 146. Intake air passage 146 can communicate with othercylinders of engine 10 in addition to cylinder 14. In some embodiments,one or more of the intake passages may include a boosting device such asa turbocharger or a supercharger. For example, FIG. 1 shows engine 10configured with a turbocharger including a compressor 174 arrangedbetween intake passages 142 and 144, and an exhaust turbine 176 arrangedalong exhaust passage 148. Compressor 174 may be at least partiallypowered by exhaust turbine 176 via a shaft 180 where the boosting deviceis configured as a turbocharger. However, in other examples, such aswhere engine 10 is provided with a supercharger, exhaust turbine 176 maybe optionally omitted, where compressor 174 may be powered by mechanicalinput from a motor or the engine. A throttle 162 including a throttleplate 164 may be provided along an intake passage of the engine forvarying the flow rate and/or pressure of intake air provided to theengine cylinders. For example, throttle 162 may be disposed downstreamof compressor 174 as shown in FIG. 1, or alternatively may be providedupstream of compressor 174.

Exhaust passage 148 can receive exhaust gases from other cylinders ofengine 10 in addition to cylinder 14. Exhaust gas sensor 128 is showncoupled to exhaust passage 148 upstream of emission control device 178.Sensor 128 may be selected from among various suitable sensors forproviding an indication of exhaust gas air/fuel ratio such as a linearoxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), atwo-state oxygen sensor or EGO (as depicted), a HEGO (heated EGO), aNOx, HC, or CO sensor, for example. Emission control device 178 may be athree way catalyst (TWC), NOx trap, various other emission controldevices, or combinations thereof.

The temperature of the exhaust system may be maintained at or below apredetermined temperature, or within a predetermined temperature range,by exhaust temperature control routine 200, as further explained in FIG.2. The predetermined temperature may, for example, be an upper limittemperature beyond which the reliability of the exhaust systemcomponents may be compromised. Exhaust temperature may be estimated byone or more temperature sensors (not shown) located in exhaust passage148. Alternatively, exhaust temperature may be inferred based on engineoperating conditions such as speed, load, air-fuel ratio (AFR), sparkretard, etc. Further, exhaust temperature may be computed by one or moreexhaust gas sensors 128. It may be appreciated that the exhaust gastemperature may alternatively be estimated by any combination oftemperature estimation methods listed herein.

Each cylinder of engine 10 may include one or more intake valves and oneor more exhaust valves. For example, cylinder 14 is shown including atleast one intake poppet valve 150 and at least one exhaust poppet valve156 located at an upper region of cylinder 14. In some embodiments, eachcylinder of engine 10, including cylinder 14, may include at least twointake poppet valves and at least two exhaust poppet valves located atan upper region of the cylinder.

Intake valve 150 may be controlled by controller 12 via actuator 152.Similarly, exhaust valve 156 may be controlled by controller 12 viaactuator 154. During some conditions, controller 12 may vary the signalsprovided to actuators 152 and 154 to control the opening and closing ofthe respective intake and exhaust valves. The position of intake valve150 and exhaust valve 156 may be determined by respective valve positionsensors (not shown). The valve actuators may be of the electric valveactuation type or cam actuation type, or a combination thereof. Theintake and exhaust valve timing may be controlled concurrently or any ofa possibility of variable intake cam timing, variable exhaust camtiming, dual independent variable cam timing or fixed cam timing may beused. Each cam actuation system may include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT) and/or variable valve lift (VVL)systems that may be operated by controller 12 to vary valve operation.For example, cylinder 14 may alternatively include an intake valvecontrolled via electric valve actuation and an exhaust valve controlledvia cam actuation including CPS and/or VCT. In other embodiments, theintake and exhaust valves may be controlled by a common valve actuatoror actuation system, or a variable valve timing actuator or actuationsystem.

Cylinder 14 can have a compression ratio, which is the ratio of volumeswhen piston 138 is at bottom center to top center. Conventionally, thecompression ratio is in the range of 9:1 to 10:1. However, in someexamples where different fuels are used, the compression ratio may beincreased. This may happen for example when higher octane fuels or fuelswith higher latent enthalpy of vaporization are used. The compressionratio may also be increased if direct injection is used due to itseffect on engine knock.

In some embodiments, each cylinder of engine 10 may include a spark plug192 for initiating combustion. Ignition system 190 can provide anignition spark to combustion chamber 14 via spark plug 192 in responseto spark advance signal SA from controller 12, under select operatingmodes. However, in some embodiments, spark plug 192 may be omitted, suchas where engine 10 may initiate combustion by auto-ignition or byinjection of fuel as may be the case with some diesel engines.

In some embodiments, each cylinder of engine 10 may be configured withone or more fuel injectors for providing fuel thereto. As a non-limitingexample, cylinder 14 is shown including two fuel injectors 166 and 170.Fuel injector 166 is shown coupled directly to cylinder 14 for injectingfuel directly therein in proportion to the pulse width of signal FPW-1received from controller 12 via electronic driver 168. In this manner,fuel injector 166 provides what is known as direct injection (hereafterreferred to as “DI”) of fuel into combustion cylinder 14. While FIG. 1shows injector 166 as a side injector, it may also be located overheadof the piston, such as near the position of spark plug 192. Such aposition may improve mixing and combustion when operating the enginewith an alcohol-based fuel due to the lower volatility of somealcohol-based fuels. Alternatively, the injector may be located overheadand near the intake valve to improve mixing. Fuel may be delivered tofuel injector 166 from high pressure fuel system-1 172 including a fueltank, fuel pumps, and a fuel rail. Alternatively, fuel may be deliveredby a single stage fuel pump at lower pressure, in which case the timingof the direct fuel injection may be more limited during the compressionstroke than if a high pressure fuel system is used. Further, while notshown, the fuel tank may have a pressure transducer providing a signalto controller 12.

Fuel injector 170 is shown arranged in intake passage 146, rather thanin cylinder 14, in a configuration that provides what is known as portinjection of fuel (hereafter referred to as “PFI”) into the intake portupstream of cylinder 14. Fuel injector 170 may inject fuel in proportionto the pulse width of signal FPW-2 received from controller 12 viaelectronic driver 171. Fuel may be delivered to fuel injector 170 byfuel system-2 173 including a fuel tank, a fuel pump, and a fuel rail.Note that a single driver 168 or 171 may be used for both fuel injectionsystems, or multiple drivers, for example driver 168 for fuel injector166 and driver 171 for fuel injector 170, may be used, as depicted.

Fuel may be delivered by both injectors to the cylinder during a singlecycle of the cylinder. For example, each injector may deliver a portionof a total fuel injection that is combusted in cylinder 14. Further, thedistribution and/or relative amount of fuel delivered from each injectormay vary with operating conditions, such as engine load, knock, andexhaust temperature, such as described herein below. The relativedistribution of the total injected fuel among injectors 166 and 170 maybe referred to as an injection type. For example, injecting all of thefuel for a combustion event via injector 166 may be an example of afirst injection type; injecting all of the fuel for a combustion eventvia injector 170 may be an example of a second injection type; injectingtwo-thirds of the fuel for a combustion event via injector 166 and theother third of the fuel via injector 170 may be an example of a thirdinjection type; injecting a third of the fuel for a combustion event viainjector 166 and the other two-thirds of the fuel via injector 170 maybe an example of a fourth injection type. Note that these are merelyexamples of different injection types, and various other types ofinjection and delivery may be used, and further the approach may beapplied to more than two injectors as well. Additionally, it should beappreciated that port injected fuel may be delivered during an openintake valve event, closed intake valve event (e.g., substantiallybefore the intake stroke), as well as during both open and closed intakevalve operation. Similarly, directly injected fuel may be deliveredduring an intake stroke, as well as partly during a previous exhauststroke, during the intake stroke, and partly during the compressionstroke, for example. As such, even for a single combustion event,injected fuel may be injected at different timings from a port anddirect injector. Furthermore, for a single combustion event, multipleinjections of the delivered fuel may be performed per cycle. Themultiple injections may be performed during the compression stroke,intake stroke, or any appropriate combination thereof.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine. As such each cylinder may similarly include its own set ofintake/exhaust valves, fuel injector(s), spark plug, etc.

Fuel injectors 166 and 170 may have different characteristics. Theseinclude differences in size, for example, one injector may have a largerinjection hole than the other. Other differences include, but are notlimited to, different spray angles, different operating temperatures,different targeting, different injection timing, different spraycharacteristics, different locations etc. Moreover, depending on thedistribution ratio of injected fuel among injectors 170 and 166,different effects may be achieved.

Fuel tanks in fuel systems 172 and 173 may hold fuel with different fuelqualities, such as different fuel compositions. These differences mayinclude different alcohol content, different octane, different heats ofvaporization, different fuel blends, and/or combinations thereof etc.One example of fuels with different heats of vaporization could includegasoline as a first substance with a lower heat of vaporization andethanol as a second substance with a greater heat of vaporization. Inanother example, the engine may use gasoline as a first substance and analcohol containing fuel blend such as E85 (which is approximately 85%ethanol and 15% gasoline) or M85 (which is approximately 85% methanoland 15% gasoline) as a second substance. Other feasible substancesinclude water, a mixture of alcohol and water, a mixture of alcoholsetc. In still another example, both fuels may be alcohol blends withvarying alcohol composition wherein the first fuel may be a gasolinealcohol blend with a lower concentration of alcohol than a gasolinealcohol blend of a second fuel with a greater concentration of alcohol,such as E10 (which is approximately 10% ethanol) as a first fuel and E85(which is approximately 85% ethanol) as a second fuel. Additionally, thefirst and second fuels may also differ in other fuel qualities such as adifference in temperature, viscosity, octane number, etc.

Moreover, fuel characteristics of one or both fuel tanks may varyfrequently. In one example, a driver may refill fuel tank 172 with E85one day, and E10 the next, and E50 the next, while fuel tank 174 mayhave gasoline one day, and E10 the next, and gasoline the next. The dayto day variations in tank refilling can thus result in frequentlyvarying fuel compositions of each of the fuels in tanks 172 and 174,thereby affecting the fuel compositions and/or fuel qualities deliveredby injectors 166 and 170, respectively. The differences in fuelcomposition and/or quality between injectors 166 and 170 may hereon bereferred to as fuel type. Also, the fuel types may be separatelydelivered to the combustion chamber, or mixed before delivery to thecombustion chamber.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor unit 106, input/output ports 108, an electronic storagemedium for executable programs and calibration values shown as read onlymemory chip 110 in this particular example, random access memory 112,keep alive memory 114, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 122; engine coolant temperature (ECT)from temperature sensor 116 coupled to cooling sleeve 118; a profileignition pickup signal (PIP) from Hall effect sensor 120 (or other type)coupled to crankshaft 140; throttle position (TP) from a throttleposition sensor; and absolute manifold pressure signal (MAP) from sensor124. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Manifold pressure signal MAP from a manifold pressure sensormay be used to provide an indication of vacuum, or pressure, in theintake manifold.

Engine 10 may further include a fuel vapor purging system (not shown)for storing and purging fuel vapors to the intake manifold of the enginevia vacuum generated in the intake manifold.

FIG. 2 describes a control system routine for controlling exhausttemperature for an internal combustion engine affecting engine operatingparameters, a fuel type delivered, as well as the type of injectionused, and combinations thereof. Specifically, the routine estimates anexhaust temperature, and suitably adjusts engine operation settings suchas spark timing, air flow, air-fuel ratio or equivalence ratio, boostlevel, for example, and further selects a combination of fuel andinjection type to be used so as to allow maintenance of exhausttemperature values within a prescribed operating range.

At 202 the engine operating conditions are identified. These include,but are not limited to, engine temperature, engine coolant temperature,engine speed, manifold pressure, air-fuel ratio, equivalence ratio,cylinder air amount, feedback from a knock sensor, desired engine outputtorque, spark timing, barometric pressure, etc.

Based on the identified operating conditions, at 204, a fuel type andinjection type are selected. In one embodiment, two different fuels maybe used for the two injectors wherein one may be port injected gasolineand the other may be direct injected E85. In one aspect of thisembodiment, if the estimated operating conditions indicate an air-fuelratio below a desired value, the control system may increase portinjection of gasoline to allow the desired air-fuel ratio to bemaintained. The bandwidth of adjustments, pulse width limits, andvarious other factors can influence which fuel(s) is adjusted and therelative amount of the adjustment. In another aspect, if the operatingconditions indicate that the engine is operating close to knock limits,the control system may increase direct injection of E85 at medium tohigh operating loads to take advantage of the charge cooling effect ofalcohol, while deferring usage of a port injected gasoline to lowoperating loads where knock may less constraining. In another aspect, ifthe operating conditions indicate that the engine is operating close toknock limits, the control system may maintain some port injection ofgasoline and a reduced injection of E85, while also retarding sparktiming from peak torque timing.

At 206, the exhaust temperature (T_(exh)) may be measured or estimated.The estimated temperature may reflect, for example, a catalysttemperature. Exhaust temperature may be estimated as a function ofengine operating parameters such as engine rotational speed, engineload, amount of spark retard, air-fuel ratio, and the like, and mayadditionally be based on the current fuel type, injection type, and/orother fuel delivery characteristics.

In one embodiment, engine 10 may be a “flex-fuel” engine wherein asingle fuel delivery method, specifically port fuel injection, may beemployed for the delivery of a fuel blend that may vary depending on theblend of fuel added to fill the fuel tank. In such a scenario, the heatof vaporization of either fuel may be chiefly provided by the metalsurfaces of the intake valves and ports of the port fuel injector,thereby making the effect of a fuel type on the exhaust temperaturenegligible. Therefore here, exhaust temperature may be primarilyestimated as a function of the engine operating parameters.

In another embodiment, engine 10 may use both a direct injector and aport fuel injector in each cylinder for the delivery of a single type offuel, such as gasoline. In such a scenario, the effect of injection typeon exhaust temperature estimation may be negligible given that the heatof vaporization of gasoline is relatively low. In one analysis of suchan engine, a back-to-back comparison of port injected gasoline versusdirect injected gasoline reveals an air-fuel ratio difference of 0.1 foran equivalent exhaust temperature. Therefore, in such an enginearrangement, exhaust temperature may again be estimated as a function ofthe previously described engine operating parameters.

In still another embodiment, engine 10 may operate with both a multitudeof fuel and delivery types. For example, a direct injection of analcohol fuel blend, and a port injection of gasoline, where again thefuel blend can vary depending on the blend of fuel added to the tank. Insuch a situation, the high heat of vaporization of the directly injectedalcohol based fuel, in conjunction with the fuel spray characteristics,may result in large exhaust temperature effects. Specifically, thedirectly injected fuel spray may not substantially contact metalsurfaces thereby allowing the heat of vaporization to be provided by theair fuel mixture. Analyses indicate a significant potential differencein the enrichment affects on exhaust temperature upon comparison ofexhaust temperatures for directly injected gasoline versus directlyinjected E85.

Given the dependence of exhaust temperature on fuel type, and further onfuel injection type, the exhaust temperature may be estimated by one ofa multitude of mapping methods, based on the prevalent combination offuel and fuel delivery type. In one example, a simple on/off mappingmethod may be used. This may be selected when the engine operates in twodistinct modes, for example a DI of E85 or a PFI of gasoline. In such asituation, the control system may estimate the exhaust temperatureindependently in each mode as a function of engine operating parameterssuch as engine speed, engine load, air-fuel ratio, equivalence ratio,and the like.

In another example, exhaust temperature may be estimated by anothermapping method. This may be selected when the engine operates withvarying configurations of fuel type and injection type, such as DI ofE85 and PFI of gasoline wherein the amount of fuel injected at any givencycle may be an appropriate distribution of fuel between both injectors.Under certain operating conditions, engine 10 may be injected with fuelthat has a 10% component of ethanol from the direct injector and a 90%component of gasoline from the port fuel injector. In another set ofoperating conditions, engine 10 may be injected with fuel that has a 20%component of ethanol from the direct injector and an 80% component ofgasoline from the port fuel injector, and so on. It may be appreciatedthat the injection ratios may further be dynamically altered as changesarise in engine operating conditions, fuel levels, etc. Under suchdynamically changing conditions, the engine may be mapped for a widerange of possible combinations. A look-up table configured to estimatean exhaust temperature for a given combination of fuel type andinjection type may thus be used. Alternatively, a curve-fit mappingmethod may be employed for the above described dynamically changingconditions wherein a certain degree of mapping data in conjunction witha certain degree of simulation data may provide an equation to relateexhaust temperature to a fuel type, injection type, and variouscombinations thereof.

At 208, the estimated T_(exh) is compared to a predetermined limit. Thepredetermined temperature limit may be, for example, an upper limittemperature beyond which the exhaust system components may degrade.Alternatively, the limit may be set outside a predetermined temperaturerange within which the catalyst function may be maintained.Specifically, prolonged periods of catalyst over-temperature conditionsmay degrade the catalyst materials. While catalyst over-temperatureconditions arising during engine start-up and warming conditions may beaddressed by adjusting parameters such as spark timing, enrichment,boost, etc. to decrease exhaust temperature, over-temperature conditionsat non-starting conditions may be addressed differently. Thus,over-temperature conditions that occur at non-starting conditions, suchas during towing and/or other high load conditions over varying terrain,may utilize a multitude of adjustments. These may include adjustments tocombinations of fuel type, injection type, boost level and spark timing,for example.

For example, if exhaust over-temperature conditions are detected at 208,at 210, the control system may proceed to appropriately perform avariety of adjustments, as elaborated herein below. The fuel combustioncharacteristics, heats of vaporization, fuel availabilities, chargecooling abilities of the prevalent fuels, and the prevalent engineoperating conditions may be weighed in when selecting the appropriatefuel type, fuel injection type, and distribution ratio of fuel among themultiple injectors

In one example, engine 10 may use a PFI injector to inject gasoline(hereafter also referred to as “fuel type 1”) and a DI fuel injector toinject E85 or an ethanol blend (hereafter also referred to as “fuel type2”). When the system senses exhaust temperature over-shoot, the controlsystem may respond by increasing injection of the second fuel type,while at the same time decreasing injection of the first fuel type(gasoline), while maintaining a desired air-fuel equivalence ratio ofthe exhaust, to reduce exhaust over-temperature. Such operation may betaken under selected conditions, such as when sufficient storage of thesecond fuel type is available, and when the alcohol concentration of thesecond fuel type is above a threshold, such as E50 (50% ethanol and 50%gasoline). By taking advantage of the charge cooling properties of thealcohol based fuel while not disturbing the air-fuel equivalence ratio,the exhaust over-temperature may be addressed without compromisingengine performance.

Charge cooling from the alcohol based fuel may have both a direct effecton exhaust temperature, and an indirect effect on exhaust temperaturedue to improved knock limits. For example, when operating primarily witha port injection of gasoline, knocking may be addressed by retarding thespark timing. However, while this approach allows knocking to bereduced, it increases exhaust temperature. Alternatively, whenaddressing knock by increasing direct injection of ethanol, for example,spark timing can be advanced back to a peak torque timing, thus reducingexhaust temperature by both reducing spark retard, and by changing thecomposition of the combusted fuel.

Thus in one example, the system may address knocking and exhausttemperature increases when operating with a spark retard, by advancingthe spark timing in addition to increasing injection of the second fueltype and decreasing injection of the first fuel type. The amount of thespark timing advance may be based on the amount of increased injectionof the second fuel. Furthermore, the amount of spark timing advance andthe amount of increased injection of the second fuel may in turn bebased on the storage level of the first or second fuels. In one aspectof this example, if the system detects the level of E85 in thecorresponding fuel tank 172 falling below a threshold level, the systemmay adjust knock levels and exhaust over-temperature conditions byincreasing direct injection of E85 by a smaller amount and retardingspark timing by a larger amount. In another aspect of this example, ifthe system detects the level of gasoline in the corresponding fuel tank173 falling below a threshold level, the system may adjust knock levelsand exhaust over-temperature conditions by increasing direct injectionof E85 by a larger amount and advancing spark timing by a correspondingamount, thus using less, or no, spark retard. In this way, the systemmay coordinate use of spark retard and gasoline/E85 usage to addressknock and exhaust over-temperature, while balancing consumption of thevarious fuels.

In another example, in addition to increasing injection of the secondfuel and reducing injection of the first fuel, the exhaust mixture maybe enriched to allow exhaust temperature control to ensue (theenrichment may be achieved by increasing the first fuel type, the secondfuel type, or both). In yet another example, in further addition toincreasing injection of the second fuel, and reducing injection of thefirst fuel, boost levels may be reduced after enriching the exhaustmixture. In still another example, the injection of both first andsecond fuels may be increased followed by exhaust mixture enrichment tocontain exhaust temperature rises.

Next, the control system allows the exhaust temperature to be furtherregulated responsive to current and potential fuel constraints. At 212,the system checks for constraints on fuel type 2 wherein furtheraddition of fuel type 2 may not be possible, or not desirable. In theexample cited above, an increasing injection of fuel type 2 responsiveto an estimated exhaust over-temperature proceeds until reaching aconstraint on the second fuel. This may arise when directly injectedfuel type 2 reaches a 100% ethanol limit whereafter further adjustmentsto exhaust temperature may not be achieved by increasing the amount ofDI. Alternative constraints on fuel type 2 may include, for example, thedirect injector running at a maximum duty cycle, depleting storage offuel type 2 in fuel tank 172, drop in separation performance of a fuelseparator located in fuel tank 172 or a combination thereof. Furtherstill, a constraint may be imposed when it is desirable to curtailconsumption of the alcohol based fuel and defer usage to an alternatetime, for example during higher engine loads where knock constraints aremore prevalent.

If at 212, constraints on fuel type 2 are identified, then at 214, thecontrol system may shift to adjusting fuel type 1 in response to thecatalyst over-temperature. In the embodiment described above, if forexample the direct injector has reached the 100% ethanol limit, then theengine may operate on an increased port injection of gasoline. However,this adjustment may alter the air-fuel equivalence ratio. In addition toincreasing the injection of fuel type 1, the controller may enhanceenrichment, reduce boost levels, retard spark timing, or any combinationthereof. If no constraint on fuel type 2 is identified at 212, theroutine progresses directly to 216.

An increasing injection of fuel type 1 responsive to an estimatedexhaust over-temperature then proceeds until a constraint on fuel type 1is identified at 216. In the described embodiment, constraints on fueltype 1 may arise, for example if the port injector is running at amaximum duty cycle, if the engine approaches a predetermined smokelimit, or if the AFR reaches a lowest permissible boundary (i.e.,becomes too rich). If no constraint is identified, at 220 the routineproceeds to maintain the engine's status. However, in the event whereconsecutive checks have identified constraints on both fuel types 1 and2, the control system may regulate the exhaust temperature by adjustingalternate engine operating parameters, for example by reducing boostlevels, at 218. Alternative adjustments may involve valve operationssuch as valve timing, valve lift, and duration of valve opening orclosing, shifting of transmission, etc.

By adjusting engine operations, such as spark timing, boost level,wastegate position, bypass valve position etc, based on exhausttemperature and a desired operational temperature range, and furthertaking into account fuel composition, and the delivery of a first andsecond fuel, it may be possible to monitor and control exhausttemperature variations while allowing for interactions between fuel typeand injection type. For example, as an exhaust temperature limit isreached, fuel injection may be adjusted responsive to variation in theamount of ethanol in a fuel blend that is directly injected into theengine, as well as responsive to the relative distribution of the totalamount of injected fuel from among a plurality of injectors for thecylinder, while maintaining a desired air-fuel or equivalence ratio.Such compensations may allow the exhaust temperature to be maintainedwithin limits at high power levels with reduced enrichment, therebyimproving fuel consumption, exhaust emissions, and component temperatureprotection at high power levels, even when the fuel type and injectiontype changes dynamically during engine operation.

Note that the example control and estimation routines included hereincan be used with various system configurations. The specific routinesdescribed herein may represent one or more of any number of processingstrategies such as event-driven, interrupt-driven, multi-tasking,multi-threading, and the like. As such, various actions, operations, orfunctions illustrated may be performed in the sequence illustrated, inparallel, or in some cases omitted. Likewise, the order of processing isnot necessarily required to achieve the features and advantages of theexample embodiments described herein, but is provided for ease ofillustration and description. One or more of the illustrated actions,functions, or operations may be repeatedly performed depending on theparticular strategy being used. Further, the described operations,functions, and/or acts may graphically represent code to be programmedinto computer readable storage medium in the control system

Further still, it should be understood that the systems and methodsdescribed herein are exemplary in nature, and that these specificembodiments or examples are not to be considered in a limiting sense,because numerous variations are contemplated. Accordingly, the presentdisclosure includes all novel and non-obvious combinations of thevarious systems and methods disclosed herein, as well as any and allequivalents thereof.

1-22. (canceled)
 23. An engine method, comprising injecting first andsecond fuels to an engine cylinder from first and second injectors,respectively, the second fuel having a greater alcohol amount than thefirst fuel; during select conditions, adjusting spark timing,enrichment, and boost level responsive to catalyst over-temperatureconditions that degrade catalyst materials; and during towingconditions, increasing the second fuel injection responsive the catalystover-temperature conditions.
 24. The method of claim 23 furthercomprising reducing injection of the first fuel while increasinginjection of the second fuel to maintain a desired air-fuel equivalenceratio of an exhaust from the cylinder, while reducing exhaustover-temperature.
 25. The method of claim 24 wherein first the secondfuel is increased and the first fuel is reduced responsive to theover-temperature condition, and then the exhaust air-fuel equivalenceratio is enriched to control exhaust temperature below a thresholdvalue.
 26. The method of claim 25 further comprising reducing a boostlevel after enriching the exhaust air-fuel equivalence ratio to controlexhaust temperature below the threshold value.
 27. The method of claim23 where the first fuel includes gasoline and the second fuel includesE85.
 28. The method of claim 24 where said reducing injection of thefirst fuel while increasing injection of the second fuel proceeds untilreaching a constraint on the second fuel, and then the exhaust air-fuelequivalence ratio is enriched to control exhaust temperature below athreshold value.
 29. The method of claim 24 where said reducinginjection of the first fuel while increasing injection of the secondfuel proceeds until depleting storage of the second fuel, and then theexhaust air-fuel equivalence ratio is enriched to control exhausttemperature below a threshold value.
 30. The method of claim 29 wheresaid enrichment of the exhaust air-fuel equivalence ratio proceeds untilreaching a constraint on the first fuel, and then a boost level isreduced to control exhaust temperature below the threshold value. 31.The method of claim 29 where said enrichment of the exhaust air-fuelequivalence ratio proceeds until depleting storage of the first fuel,and then a boost level is reduced to control exhaust temperature belowthe threshold value.
 32. The method of claim 23 further comprisingvarying delivery of the first and second fuels during engine operation,and where alcohol composition of at least one of the first and secondfuels varies.
 33. A method of operating an engine in a vehicle, themethod comprising delivering a first fuel to a cylinder of the enginefrom a first, port, injector; delivering a second fuel to the cylinderof the engine from a second, direct, injector, where the second fuel hasa greater concentration of alcohol than the first fuel; during a firstmode, increasing injection of the second fuel while decreasing injectionof the first fuel responsive to an exhaust over-temperature condition,the first mode including non-starting towing operation; during a secondmode, enriching an exhaust mixture from the cylinder responsive to theexhaust over-temperature condition, the over-temperature conditionincluding catalyst over-temperature conditions that degrade catalystmaterials; and during a third mode, reducing boosting of the engineresponsive to the exhaust over-temperature condition.
 34. The method ofclaim 33 further comprising selecting among the first and second modesbased on a storage level of the first and second fuels.
 35. The methodof claim 33, where the first mode is selected when an amount of thesecond fuel stored is greater than a threshold level, and where thesecond fuel has an alcohol concentration greater than a threshold value.36. The method of claim 33 further comprising selecting among the firstand second modes responsive to constraints on said first and secondfuels, and where a third mode including reducing boost in response tothe exhaust over-temperature condition when both the first and secondfuels are constrained, where the first mode is selected whenavailability of the second fuel is above a first threshold and thesecond mode is selected when availability of the second fuel is belowthe first threshold.
 37. An engine method, comprising delivering a firstfuel to an engine cylinder; delivering a second fuel to the cylinder,where the second fuel has a greater concentration of alcohol than thefirst fuel; operating with spark retarded from a peak torque timing; andincreasing the second fuel injection while decreasing the first fuelinjection responsive to exhaust over-temperature conditions andadvancing spark timing, a spark advance amount based on an amount of thesecond fuel injection increase.
 38. The method of claim 37 wherein thesecond fuel injection increase amount is based on a storage level of thefirst or second fuel.